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Abstract

Background

Findings from observational studies suggest that sex hormone-binding globulin (SHBG)
and endogenous sex hormones may be mediators of the putative relation between coffee
consumption and lower risk of type 2 diabetes. The objective of this study was to
evaluate the effects of caffeinated and decaffeinated coffee on SHBG and sex hormone
levels.

Findings

After a two-week run-in phase with caffeine abstention, we conducted an 8-week parallel-arm
randomized controlled trial. Healthy adults (n = 42) were recruited from the Boston
community who were regular coffee consumers, nonsmokers, and overweight. Participants
were randomized to five 6-ounce cups of caffeinated or decaffeinated instant coffee
or water (control group) per day consumed with each meal, mid-morning, and mid-afternoon.
The main outcome measures were SHBG and sex hormones [i.e., testosterone, estradiol,
dehydroepiandrosterone sulfate].

No significant differences were found between treatment groups for any of the studied
outcomes at week 8. At 4 weeks, decaffeinated coffee was associated with a borderline
significant increase in SHBG in women, but not in men. At week 4, we also observed
several differences in hormone concentrations between the treatment groups. Among
men, consumption of caffeinated coffee increased total testosterone and decreased
total and free estradiol. Among women, decaffeinated coffee decreased total and free
testosterone and caffeinated coffee decreased total testosterone.

Conclusions

Our data do not indicate a consistent effect of caffeinated coffee consumption on
SHBG in men or women, however results should be interpreted with caution given the
small sample size. This is the first randomized trial investigating the effects of
caffeinated and decaffeinated coffee on SHBG and sex hormones and our findings necessitate
further examination in a larger intervention trial.

Keywords:

Coffee; Sex hormones; Randomized trial

Introduction

Coffee consumption has been consistently associated with a lower risk of type 2 diabetes
(T2DM), but the underlying mechanisms remain unclear. Data from observational studies
suggest that sex hormone-binding globulin (SHBG) and endogenous sex hormones may modulate
glycemia and risk of T2DM in men and women
[1-5]. Caffeinated coffee consumption has been found to be associated with higher SHBG
levels in data from cross-sectional studies in women
[2,6-8]. It has been hypothesized that SHBG may be an intermediate pathway to explain the
putative effect of coffee on lowering the risk of T2DM
[3].

We conducted an 8-week parallel-arm randomized trial to determine the effects of caffeinated
and decaffeinated coffee on risk factors for T2DM. To our knowledge, there have been
no randomized trials to investigate this research question.

Participants and methods

The details of this study have been previously described
[9]. Briefly, eligible participants were overweight (body mass index 25–35 kg/m2), nonsmoking men and women aged 18 years or older who habitually consumed coffee
(at least two cups per day). Exclusion criteria included the presence of diabetes,
heart disease, stroke, hypertension, alcoholism or substance abuse, abnormal hepatic
or renal function, gastro-esophageal reflux disease, a medical history of ulcers,
or women planning a pregnancy or breastfeeding. Exclusions were made for individuals
on medications for chronic health conditions.

Sixty-five adults were screened of which 11 were ineligible and 9 withdrew from the
study prior to randomization. Three individuals did not continue the study after the
baseline visit and were not included in the current analysis. The final study population
included 14 men and 28 women. The study was approved by the institutional review boards
of the Beth Israel Deaconess Medical Center and the Harvard School of Public Health
and all participants provided written informed consent. The clinical trial registration
number is NCT00305097.

After two weeks of caffeine abstention, participants attended the baseline visit in
the morning after fasting overnight for at least 12 hours. Participants were randomized
to either caffeinated coffee, decaffeinated coffee, or no coffee (control) treatment
groups. Treatment assignments for the coffee arms were blinded to the study participants,
investigators, and laboratory staff. Participants in the coffee treatment groups were
given five two-gram portions of instant coffee per day (caffeinated or decaffeinated
Nestlé’s Taster’s Choice®) to be mixed with approximately 6 ounces of boiling water
and consumed with every meal and mid-morning and mid-afternoon. Non-caloric sweetener
and non-dairy creamer were also provided. Participants in the control group were instructed
to drink the equivalent amount of water at the same intervals throughout the day.

At baseline, week 4, and week 8, the study visits included a physical examination,
anthropometric measurements, and a fasting blood draw. Sex hormone-binding globulin
and all other endogenous sex hormones were measured via Access chemiluminescent immunoassay
(Beckman Coulter, Fullerton, CA). Free testosterone and free estradiol were calculated
using the Sodergard formula which is based on the law of mass action and assumptions
of equilibrium binding
[10].

All analyses were performed separately for men and women. Using general linear models,
we evaluated the change from baseline in SHBG, testosterone (total and free), estradiol
(total and free), testosterone to estradiol ratio, and DHEAs regressed on treatment
group as a main effect with baseline log values of the dependent variable and age
as additional covariates. Both covariates were grand mean centered to improve the
interpretability of the estimates. Because SHBG and sex hormones did not follow a
normal distribution, the variables were log-transformed and subsequently back-transformed
to yield geometric means. Differences between caffeinated and decaffeinated coffee
compared with the control group were based on linear contrasts. The adjusted geometric
means with standard errors were reported by treatment, and 95% confidence intervals
(CI) were computed. In addition, we calculated the difference between the treatment
groups versus control for change from baseline. This yielded a ratio (or percentage
when subtracting the value one and multiplying by 100), given the principles of logged
numbers.

Statistical significance was evaluated at an alpha level of 0.05. The Statistical
Analysis System version 9.1.3 was used for all analyses (SAS Institute, Cary, NC).

Results

In men, mean concentrations were 25.3 nmol/L for SHBG, 411.3 ng/dL for testosterone,
8.0 ng/dL for free testosterone, 24.9 pg/mL for estradiol, 0.5 pg/mL for free estradiol,
and 194.9 ng/mL for DHEAs. In women, mean concentrations were 51.5 nmol/L for SHBG,
35.9 ng/dL for testosterone, 0.5 ng/dL for free testosterone, 63.2 pg/mL for estradiol,
1.0 pg/mL for free estradiol, and 133.7 ng/mL for DHEAs. The baseline characteristics
of the study population are shown in Table
1 according to treatment group. Ten women were postmenopausal. The average age was
40 years for both men (range 23–72 years) and women (18–69 years). Mean body mass
index was 30.3 kg/m2 for men and 29.0 kg/m2 for women. All participants included in the analysis population completed the 8-week
trial with the exception of one female participant who discontinued after the 4-week
visit. Four non-serious adverse events were reported during the course of the intervention.

Adjusted geometric means followed by percent change from baseline estimates for the
endpoints are shown for men (Table
2) and women (Table
3). At the final study visit (week 8) there were no significant differences for any
of the outcomes. In addition, we did not observe an effect of coffee intake on SHBG
levels in men, although a borderline significant increase for decaffeinated coffee
was observed among women [difference in change from baseline (CFB): 38%; 95% CI: 1%,
88%; p = 0.04] compared with consuming no coffee at week 4. In contrast, several significant
differences between the treatment groups were found at week 4. Among men, consumption
of caffeinated coffee increased total testosterone (CFB: 67%; 95% CI: 4%, 168%; p = 0.04)
and decreased total and free estradiol (CFB total: -47%; 95% CI: -19%, -65%; p = 0.01
and CFB free: -43%; 95% CI: -10%, -64%; p = 0.02). Among women, decaffeinated coffee
decreased total and free testosterone (CFB total: -60%; 95% CI: -24%, -79%; p = 0.01
and CFB free: -68%: 95% CI: -26%, -86%; p = 0.01) and caffeinated coffee decreased
total testosterone (p = 0.04). The ratio of testosterone to estradiol, a potential
marker for aromatase activity, was significantly increased in men in the caffeinated
coffee group at week 4 (CFB: 189%; 95% CI: 39%, 502%; p = 0.01) whereas no significant
differences were observed for women. We did not observe any significant effects for
DHEAs in either men or women.

Table 2.Sex hormone-binding globulin and endogenous sex hormones by treatment group at week
4 and week 8 in men

Table 3.Sex hormone-binding globulin and endogenous sex hormones by treatment group at week
4 and week 8 in women

Discussion

In this randomized controlled trial with a caffeinated and decaffeinated coffee intervention,
we did not find evidence of a consistent effect on SHBG levels in overweight men or
women. All significant effects for SHBG and the hormone measurements were limited
to the week 4 visit with no significant effects observed at the time of the final
week 8 visit.

Previous studies on coffee or caffeine consumption in relation to SHBG and sex hormone
concentrations all had a cross-sectional design and have been almost exclusively conducted
in women. The Additional file
1: Table S1 shows the characteristics and findings from these studies. Our results
for caffeinated coffee and SHBG are consistent with two previous cross-sectional studies
[11,12] which did not find an association with consumption of caffeine or caffeinated coffee,
whereas other studies did detect direct associations
[2,3,6-8]. In contrast to our findings for caffeinated coffee, we found slightly elevated SHBG
levels in the decaffeinated group as compared with the control group at week 4 in
women, but this observation was limited to women and not observed at week 8 and may
well represent a chance finding. Few studies have specifically studied decaffeinated
coffee, but the Women’s Health Study
[3] observed no association between decaffeinated coffee and SHBG.

Our results in women of a decrease in total testosterone levels at week 4 in both
caffeinated and decaffeinated arms are not consistent with the lack of association
between coffee consumption and testosterone in previous observational studies
[2,3,7]. We did not observe a significant effect of coffee consumption on estradiol concentrations
among the women in our trial. This finding agrees with four cross-sectional studies
that also found no association between caffeinated coffee consumption and estradiol
[3,6,7,11]. In contrast, an inverse association between coffee consumption and luteal estradiol
and luteal free estradiol was observed among premenopausal women
[2] and a direct association was observed for follicular estradiol in another study among
premenopausal women
[12]. It is currently unclear whether the discrepancy between our findings and previous
studies is due to the limited power or duration of our trial or methodological limitations
of the cross-sectional studies.

As mentioned previously, little data has been published on coffee consumption and
SHBG or sex hormones in men. Our finding that caffeinated coffee, but not decaffeinated
coffee, significantly increased total testosterone and decreased both total and free
estradiol after 4 weeks suggests that caffeine may act as an aromatase (or CYP19) inhibitor. One intervention trial
[13] found that consumption of two cups of instant coffee had no acute effect on testosterone
or estradiol concentrations after 30 minutes.

This is the first randomized controlled trial investigating the effects of caffeinated
and decaffeinated coffee on SHBG and sex hormones. Attrition was low among participants
and non-fasting blood samples measured for caffeine and its major metabolites at the
6-week visit indicated that compliance was high. Our study also had several limitations
that need to be considered. Most notably, our study has a small sample size which
may have limited our ability to detect modest effects on SHBG and sex hormone levels.
Thus, findings should be interpreted with caution and require confirmation in larger
trials. In addition, given the small sample size, stratifying analyses by menopausal
status was not appropriate. Inclusion of age in the analysis of covariance models
was an attempt to address this issue. From the evidence to date, it is clear that
heterogeneity among the observational studies in timing of the hormone measurements
in women has most likely led to some of the divergent findings. We inquired about
the last menstrual period or menopausal status, in addition to having the two follow-up
visits timed approximately four weeks apart to reduce the variation in measurement
for sex hormones by follicular and luteal cycle timing. However, it is plausible that
inadequate control for menopausal status attenuated our results given the variability
in the women’s ages and that, for example, the majority of postmenopausal women were
in the decaffeinated coffee group. In our trial, lack of finding any significant effects
at the 8-week visit for any of the measurements lends to the hypothesis that habituation
may have occurred by the time of the final 8-week visit. Longer randomized trials
with larger sample size will be necessary to elucidate the temporality of the potential
effects.

In this randomized controlled trial with caffeinated and decaffeinated coffee interventions,
we did not find evidence of a consistent effect on SHBG levels in overweight men or
women. This contrasts with the beneficial effects of coffee consumption on adiponectin
and fetuin-A levels previously reported in this trial
[9], suggesting that the SHBG level is not the major intermediate of the putative effect
of coffee consumption on a lower risk of T2DM. Our findings necessitate further examination
in a larger intervention trial of the effects of coffee on sex hormones to elucidate
if this is a potential intermediary mechanism to explain the beneficial effects observed
of coffee intake and T2DM.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

NW contributed towards the acquisition, analysis and interpretation of the data, drafted
the manuscript, and critically reviewed the manuscript. CM contributed towards the
study concept and design, the acquisition, analysis and interpretation of the data,
and critically reviewed the manuscript. ED contributed towards the analysis and interpretation
of the data, and critically reviewed the manuscript. AB contributed towards the study
concept and design, the acquisition and interpretation of the data, and critically
reviewed the manuscript. BR contributed towards the analysis and interpretation of
the data, and critically reviewed the manuscript. ER contributed towards the analysis
and interpretation of the data, and critically reviewed the manuscript. FH contributed
towards the study concept and design, the analysis and interpretation of the data,
and critically reviewed the manuscript. RVD contributed towards the study concept
and design, the acquisition, analysis and interpretation of the data, drafted the
manuscript, critically reviewed the manuscript, and obtained funding for the study.
All authors have read and approved the final manuscript.

Acknowledgements

We gratefully thank the participants of the Coffee Trial for their participation.
We also thank the General Clinical Research Center (GCRC) nurses at Beth Israel Deaconess
Medical Center for assistance with collection of the samples for this research, and
the GCRC nutritionists for conducting the dietary assessments, body composition measurements,
and dispensing of treatment. We thank the Harvard Catalyst Human Research Center Laboratory
for the timely and careful measurement of the sex hormones used for this analysis.

The research for this study was financially supported by a Harvard Catalyst Human
Research Center Laboratory Support Award, a Boston Obesity Nutrition Research Center
pilot and feasibility grant (grant#: 2005-P-000377/2), and a National Institutes of
Health - National Center for Research Resources grant M01-RR-01032 (Harvard Clinical
and Translational Science Center) and grant number UL1 RR025758. The Mantzoros Lab
is also supported by the National Institute of Diabetes and Digestive and Kidney Diseases
grants 58785, 79929 and 81913, and AG032030.

The clinical trial registration number is NCT00305097. The coffee was supplied by
Nestlé.